TFT substrate having scanning lines of metal films of columnar crystal grains

ABSTRACT

A liquid crystal display comprises a lower substrate provided with a plurality of scanning signal lines, a plurality of image signal lines perpendicularly intersecting the scanning signal lines, a plurality of thin-film transistors formed at the intersection points of the scanning signal lines and the image signal lines, and a plurality of pixel electrodes connected respectively to the thin-film transistors; an upper substrate disposed opposite to the lower substrate and provided with a common electrode opposite to the pixel electrodes; and liquid crystal layer sealed in a space formed between the lower and the upper substrate. Each of the scanning signal lines is formed so as to form the gate electrode of a corresponding thin-film transistor, the scanning signal lines are formed by processing a metal film of columnar crystal grains, and the surface of the metal film is coated with a self-aligned oxide film.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display and, moreparticularly, to a thin-film transistor drive type liquid crystaldisplay and a method of fabricating such a liquid crystal display.

A thin-film transistor drive type liquid crystal display (hereinafterreferred to as "TFT liquid crystal display") employing thin-filmtransistors (TFTs) is one known type of liquid crystal display. Thisknown TFT liquid crystal display is provided with TFTs formed on atransparent substrate to control the voltage to be applied to a liquidcrystal at each pixel, is capable of displaying sharp images, and isused widely as a display for the terminal equipment of office automationequipment and liquid crystal TV sets.

FIG. 11 is an equivalent circuit of one of the pixels of a conventionalTFT liquid crystal display. A TFT 100 is located at the intersectionpoint of a scanning signal line 101 and an image signal line 102 and isconnected to a liquid crystal capacitor 103 and a storage capacitor 104.When the TFT 100 is turned on by a signal transmitted through thescanning signal line 101, the potential of the image signal line 102 iswritten on a pixel electrode, and the liquid crystal capacitor 103 andthe storage capacitor 104 are charged. When the TFT is turned off, theliquid crystal capacitor 103 and the storage capacitor 104 hold thecharge. However, the resistances of the scanning signal line 101 and theimage signal line 102 affect picture quality. Thus, if the resistancesof the scanning signal line 101 and the image signal line 102 are large,signals are delayed and arrive at corresponding pixels at differenttimes, and hence sharp images are not displayed. Therefore, it isdesirable to form the signal lines of a wiring material having thelowest possible resistivity. An oxide film can be formed on lines ofsuch a wiring material by a well-known anodic oxidation process or thelike, and the oxide film can be used as an insulating film forinsulating the scanning signal line 101 and the image signal line 102and as a gate insulating film for the TFT 100. Aluminum is such apreferable wiring material. Wiring lines are formed by forming a film bya vacuum evaporation process, a vacuum sputtering process or the like,patterning a photoresist film formed over the film by a knownphotolithographic process to form a photoresist mask, and patterning thefilm by a wet etching process using an etchant or a dry etching processusing a chloric gas.

Sometimes, the shape of the self-aligned insulating film formed on thealuminum lines affects the conductive line yield or picture qualityadversely. For example, if the shape of the wall of the section of thescanning signal line 101 or the image signal line 102 is an overhangingshape or a nearly vertical shape, the former reduces the conductive lineyield of the image signal line 102 and the latter deteriorates theflatness of the protective film, thereby spoiling the orientation of theliquid crystal and deteriorating the picture quality. Moreover,electrostatic focusing due to the edge effect of the gate electrode isliable to cause a dielectric breakdown of the gate insulating film,which deteriorates the characteristics of the TFT. The same problemarises in the gate electrode of the TFT connected to the scanning signalline, the drain electrode connected to the image signal line and thesource electrode.

A method proposed to solve such a problem, as disclosed in JapanesePatent Laid-open No. 5-82505, etches an aluminum film with ahydrofluoric acid etchant or a nitric acid etchant to form aluminumwiring lines having a tapered cross section. However, this methoddamages the glass substrate and the insulating film, and the taper ofthe cross section of the aluminum wiring lines is excessively small andthe aluminum wiring lines have a comparatively large resistance.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aliquid crystal display having signal lines formed with an improvedconductive line yield and a protective film of improved flatness, and toprovide a method of fabricating such a liquid crystal display.

According to one aspect of the present invention, a liquid crystaldisplay comprises a lower substrate provided with a plurality ofparallel scanning signal lines, a plurality of parallel image signallines perpendicularly intersecting the scanning signal lines, aplurality of TFTs formed at the intersection points of the scanningsignal lines and the image signal lines, and a plurality of pixelelectrodes connected to the plurality of TFTs, respectively; an uppersubstrate disposed opposite to the lower substrate and provided with acommon electrode formed opposite to the pixel electrodes; and a liquidcrystal layer sandwiched between the lower substrate and the uppersubstrate; the plurality of scanning signal lines serving as the gateelectrodes of the corresponding TFTs, respectively, the scanning signallines being metal films of columnar grains, and self-aligned oxide filmsbeing formed on the metal films.

When forming conductive metal lines having an upward tapered crosssection, a conventional method utilizes the adhesion between aphotoresist film and a metal film, and another conventional method formsan intermediate layer between a photoresist film and a metal film. Thepresent invention forms conductive metal lines having an upward taperedcross section and inclined side surfaces inclined at an inclination of45° or below by utilizing the crystalline properties of the metal. Theinventors of the present invention found, through the examination ofvarious taper etching processes for etching a metal film, that thetapered shape of the cross section of conductive metal lines isdependent on the crystal grains of the material and have made thepresent invention on the basis of this finding.

Accordingly, it is a feature of the present invention to process a metalfilm to form conductive metal lines having an upward tapered crosssection utilizing the crystalline properties of the metal forming themetal film. The conductive metal lines having a tapered cross sectionand side surfaces inclined at an inclination of 45° or below andinsulating films covering the conductive metal lines and having atapered cross section and side surfaces inclined at an inclination of45° or below suppress the edge effect.

A metal material having columnar crystal grains can be obtained byadding at least one of the transition elements of groups IVa and Va toaluminum (Al) as a matrix.

An insulating film having an upward tapered cross section and sidesurfaces inclined at an inclination of 45° or below can be formed bycontrolling the nitric acid concentration and the acetic acidconcentration of a mixed etchant for etching At films prepared by mixingphosphoric acid, nitric acid and acetic acid.

The present invention is not limited in its application to a method offorming gate electrodes and gate insulator films, and further thepresent invention is effectively applicable to a method of formingscanning signal lines, image signal lines, drain electrodes extendingfrom image signal lines, and source electrodes.

The present invention is applied to processing metal films formed by asputtering process, an electron beam source evaporation process or thelike. The crystal structures of those films are different from those ofbulk metals. In general, the following crystal structures are formedunder conditions expressed by the following inequalities.

Equiaxed crystal grain Tm>T>Tr (1)

Columnar crystal grain: Tr>T>Tsd (2)

Fibrous and porous crystal grain: Tsd>T (3)

where T is the temperature of the substrate, Tm is the melting point ofthe metal, Tr is the recrystallization temperature of the metal and Tsdis the surface self-diffusion temperature of the metal.

It was confirmed through experiments, in which processes were controlledunder the above-mentioned conditions, that columnar crystal grains aremost suitable for forming conductive lines having an upward taperedcross section, that columnar crystal grains can be formed by controllingthe temperature of the substrate during the deposition of a metal filmand that the addition of at least one of the transition elements of thegroups IVa and Va to the matrix facilitates forming columnar crystalgrains.

A metal film of columnar crystal grains forms, when etched, conductivelines having an upward tapered cross section in the following way. FIG.2 depicts typically the progress of etching a metal film of columnarcrystal grains formed under conditions meeting inequality (1), and FIG.3 depicts the progress of etching a metal film of equiaxed crystalgrains formed under conditions meeting inequality (2). Basically,etching proceeds along grain boundaries and then progresses into crystalgrains as shown in FIGS. 2 and 3, in which thick lines indicateprocessions of etching at different times. When etching the metal filmof columnar crystal grains shown in FIG. 2, etching progresses alonggrain boundaries 24 at the initial stage of etching, and then etchingprogresses into crystal grains 23. It is known from close observation ofthe boundary A between the metal film and a resist film 21 that etchingprogresses only along a grain boundary 24 on one side of the crystalgrain at the initial stage of etching, and etching progresses along agrain boundary 24' on the other side of the crystal grain after etchinghas progressed into the crystal grain. This process progressescontinuously as indicated by thick lines and, finally, the metal film isetched in an upward tapered shape as indicated by a thick line 25. Inthe metal film of equiaxed crystal grains shown in FIG. 3, grainboundaries extend longitudinally (depthwise) and transversely (along thesurface). Therefore, the mode of progress of etching of the metal filmof equiaxed crystal grains is different from that of etching of themetal film of columnar crystal grains. Accordingly, it is difficult tocontrol the shape of an etched surface and the metal film of equiaxedcrystal grains cannot be etched in an upward tapered shape as shown 2.

When the metal film is etched in conductive lines having an upwardtapered cross section having inclined side surfaces inclined at aninclination θ of 45° or below, an insulating film can be stably formedover the conductive lines by an anodic oxidation process. Since ananodic forming process is carried out in a high electric field, theinsulating film is broken by electrostatic focusing on the edges of theconductive lines. Therefore, the inclination θ of 45° or below isdesirable to suppress electrostatic focusing.

FIG. 4 is a graph showing both the relation between the electrostaticfocusing coefficient and the side surface inclination θ, and thevariation of the breakdown yield of insulating films with variation ofthe side surface inclination θ. As is evident from FIG. 4, side surfaceinclinations of 45° or below tend to effectively suppress the reductionof dielectric breakdown yield of anodic oxidation coatings due toelectrostatic focusing. The resistance of a wiring line is dependent onits sectional area and increases when the side surface inclination isreduced. FIG. 4 shows the variation of the resistance of an aluminumwiring line of 10 μm in width and 400 nm in thickness with the sidesurface inclination θ of the aluminum wiring line. The resistancedecreases sharply when the side surface inclination θ decreases belowabout 20°. In view of both the suppression of the dielectric breakdownof the insulating film and the reduction of the resistance of the same,a desirable side surface inclination θ is in the range of 20° to 45°.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be understood more clearly from the following detaileddescription with reference to the accompanying drawings, wherein:

FIG. 1 is a sectional view of a typical one of the pixels of a TFTliquid crystal display in a preferred embodiment according to thepresent invention;

FIG. 2 is a diagrammatic sectional view showing a mode of progress ofetching of a metal wiring line of columnar crystal grains;

FIG. 3 is a diagrammatic sectional view showing a mode of progress ofetching of a metal wiring line of equiaxed crystal grains;

FIG. 4 is a graph showing the variations of electrostatic focusingcoefficient, dielectric breakdown yield and the resistance of wiringlines with side surface inclination;

FIGS. 5(a), 5(b) and 5(c) are sectional views of a typical workpiece atdifferent stages of a process of fabricating a TFT liquid crystaldisplay embodying the present invention;

FIGS. 6(a), 6(b) and 6(c) are sectional views of the workpiece atdifference stages of the process of fabricating the TFT liquid crystaldisplay in accordance with the present invention;

FIG. 7 is a sectional view of typical one of the pixels of a TFT liquidcrystal display in accordance with the present invention;

FIG. 8 is a triangular diagram showing the composition of etchantsemployed in the present invention;

FIG. 9 is a sectional view of a typical TFT liquid crystal display inaccordance with the present invention;

FIG. 10 is a sectional view of typical one of the pixels of a TFT liquidcrystal display in another embodiment according to the presentinvention;

FIG. 11 is a circuit diagram of an equivalent circuit representing oneof the pixels of a conventional liquid crystal display; and

FIG. 12 is a diagrammatic view of a system including a TFT liquidcrystal display in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First the constitution of a TFT liquid crystal display in accordancewith the present invention will be described with reference to FIG. 1showing a typical one of the pixels of the TFT liquid crystal display ina sectional view.

A TFT section a, a pixel electrode section b, an additional capacitorsection c, a scanning signal line and an image signal line are formed ona transparent glass substrate 20. The TFT section a comprises a gateelectrode 1, a gate insulating film 2b formed over the gate electrode 1,a second insulating film 2a, an i-type Si layer 3, a source electrode 4and a drain electrode 5. The gate electrode 1 is extended so as to mergeinto the scanning signal line 101 (FIG. 11), and the drain electrode 5is extended so as to merge into the image signal line 102 (FIG. 11). Thepixel electrode section b has a transparent pixel electrode 8. Theadditional capacitor section c comprises a capacitor electrode 7(a) ofthe same constitution as the gate electrode 1 and formed simultaneouslywith the gate electrode 1, another capacitor electrode 7(b) of the sameconstitution as the source electrode 4 and the drain electrode 5 andformed simultaneously with the source electrode 4 and the drainelectrode 5, a capacitor insulating film 11(a) of the same material asthat of the second gate insulting film 2a and formed simultaneously withthe second gate insulating film 2a, and a capacitor insulating film11(b) of the same material as that of the insulating film 2b and formedsimultaneously with the insulating film 2b. The transparent pixelelectrode 8 of the pixel electrode section b is connected through thegate insulating films 2a and 2b to the source electrode 4 of the TFTsection a, and through the capacitor insulating film 11(a) and 11(b) tothe capacitor electrode 7(b) of the additional capacitor section c.

This embodiment is featured by the gate electrode 1 of the TFT sectiona, the capacitor electrode 7(a) of the additional capacitor section c,the gate insulating film 2b formed over the gate electrode 1, and thecapacitor insulating film 11(b) respectively having upward tapered crosssections.

Metal lines having upward tapered cross sections can be formed bycontrolling the process of depositing crystal grains of the metalmaterials and the use of appropriate etchants, which will be describedin connection with a TFT liquid crystal display fabricating method in apreferred embodiment of the present invention with reference to FIGS.5(a) to 5(c), 6(a) to 6(c) and 7. Referring to FIG. 5(a), an aluminumfilm 21 is formed in a thickness in the range of 300 to 400 nm over thesurface of a transparent glass substrate 20 by sputtering underconditions meeting inequality (2), Tr>T>Tsd. Since the recrystallizationtemperature Tr of aluminum is about 423K and the surface self-diffusiontemperature Tsd of aluminum is about 1/10 of the melting point Tm of thesame, the glass substrate 20 is heated at a temperature in the range of373 to 413K when forming the aluminum film 21 by sputtering. Aphotoresist mask 22 of a desired pattern is formed by patterning aphotoresist film formed over the aluminum film 21 by an ordinaryphotolithographic process.

Then, as shown in FIG. 5(b), the workpiece is immersed in an etchant foretching the aluminum film 21 to form a gate electrode 1 of a crosssection having inclined side surfaces inclined at an inclination θ of45° or below. The etchant is prepared by mixing phosphoric acid: nitricacid, and acetic acid as a diluent.

FIG. 8 is a triangular diagram showing the dependence of the inclinationθ on the composition of the etchant, in which numerals 25, 30, 40 and 45are the inclinations of the side surfaces of gate electrodes formed byetching using etchants of different compositions, respectively. Asmentioned in connection with FIG. 4, a desirable inclination θ is 45° orbelow. From FIG. 8, the nitric acid concentration of the etchant capableof etching the aluminum film 21 to form the gate electrode 1 in a crosssection having side surfaces inclined at an inclination of 45° is 10%vol. or above. However, a nitric acid concentration exceeding 30% vol.cracks the photoresist mask. A desirable acetic acid concentration is50% vol. or above. When etchants of compositions in phosphoric acid:nitric acid: and acetic acid of 75:10:15, 70:15:15, 65:20:15 and75:25:15 are used, the inclinations are 45°, 40°, 30° and 25°,respectively. The nitric acid concentration dominates the inclination θ,which is inferred to be due to the alteration of the mechanicalproperties of the photoresist by nitric acid and the dependence of theinclination θ on the mechanical properties of the photoresist. When thenitric acid concentration is 30% vol. or below in view of thedeterioration of the photoresist, the aluminum film of columnar crystalgrains can be etched to form the gate electrode 1 of an upward taperedcross section having side surfaces inclined at a desired inclination.

Then, as shown in FIG. 5(c), a gate insulating film 2b of aluminum oxideis formed over the gate electrode 1 by an anodic oxidation process. Theanodic oxidation process may be of an ordinary system that anodizes ametal in an electrolytic solution. Practically, the aluminum oxide filmwas formed by a constant-current-constant-voltage electrolysis, in whicha platinum plate as a cathode was disposed opposite to the gateelectrode 1, the surface of which is to be anodized to form an aluminumoxide film, the platinum plate and the gate electrode 1 were immersed ina 3% tartaric acid solution as an electrolytic solution, and a formationvoltage in the range of 100 to 200 V was applied across the platinumplate and the gate electrode 1 so that the current density was 3 mA/m².The thickness of the aluminum oxide film was in the range of 150 to 200nm. The inclination of the aluminum gate electrode 1 was not changed bythe anodic oxidation process.

Then, as shown in FIG. 6(a), a transparent pixel electrode 8 of athickness in the range of 120 to 200 nm is formed by a sputteringprocess. The transparent pixel electrode 8 is a transparent conductivefilm of ITO (indium tin oxide), i.e., a NESA film.

Then, as shown in FIG. 6(b), an i-type Si layer 3, an amorphous Si layeror a polycrystalline Si layer, of a thickness of about 180 nm, is formedby depositing amorphous Si. The i-type Si layer 3 is formed subsequentlyto the formation of a SiN gate insulating film 2a and a capacitorinsulating film 11(b), in the same film forming chamber of a plasma CVDapparatus as that in which the SiN gate insulating film 2a and thecapacitor insulating film 11(b) have been formed without taking out theworkpiece from the film forming chamber. Similarly, an n+-typesemiconductor layer 3a doped with phosphorus of about 40 nm in thicknessfor ohmic contact is formed. Then, the workpiece is taken out from theplasma CVD apparatus and is subjected to a photolithographic process topattern the i-type Si layer 3 in the shape of a land on the gateelectrode 1. Then, portions of the SiN gate insulating film 2acorresponding to the transparent pixel electrode 8 and terminals, notshown, are removed by a known dry etching process so that the surface ofthe transparent pixel electrode 8 is exposed.

Then, as shown in FIG. 6(c), a source electrode 4 and a drain electrode5 are formed. Each of the source electrode 4 and the drain electrode 5has a laminated structure consisting of a first conductive film A formedin contact with the n+-type semiconductor layer 3a, and a secondconductive film B formed over the first conductive film A. Therespective first and second conductive films A of the source electrode 4and the drain electrode 5 are formed simultaneously by the same process.In this embodiment, the first conductive films A are chromium films of athickness in the range of 50 to 100 nm formed by sputtering. A chromiumfilm satisfactorily adheres to the n+-type semiconductor layer 3a. Thefirst conductive film A serves as a barrier layer that prevents thediffusion of the second conductive film B into the n+-type semiconductorlayer 3a. The first conductive film A may be formed of a metal having ahigh melting point, such as Mo, Ti, Ta or W, or a metal silicide havinga high melting point, such as MoSi₂, TiSi₂, TaSi₂ or WSi₂ instead ofchromium. The second conductive film B is an aluminum film of athickness in the range of 300 to 400 nm formed by sputtering. The secondconductive film B may be a Si-containing aluminum film or aCu-containing aluminum film.

The source electrode 4 and the drain electrode 5 are formed bypatterning the first conductive film A and the second conductive film Bby a photolithographic process as shown in FIG. 6(c). The n+-typesemiconductor layer 3a is masked with the first conductive film A, thesecond conductive film B and a photoresist mask so that the unmaskedpart of the n+-type semiconductor layer 3a is removed; that is, aportion of the n+-type semiconductor layer 3a not underlying the firstconductive film A and the second conductive film B is self-alignedlyremoved by its thickness. Although the drain electrode 4, the sourceelectrode 5 and the image signal line are formed over the gate electrode1 and the gate insulating film 2b so as to extend across the latter,defects, such as breakages, at the intersection points are reducedbecause the gate electrode 1 and the gate insulating film 2b have upwardtapered cross sections, respectively.

Then, as shown in FIG. 7, a 1 μm thick protective film 12 of SiN isformed by a plasma CVD process over the entire surface of the pixel,excluding regions corresponding to the terminals and the like.

The construction of the liquid crystal display will be describedhereinafter with reference to FIG. 9. The orientation of a liquidcrystal layer 40 is controlled by a lower orientation film 43 formedover the inner surface of a lower transparent glass substrate 41 and anupper orientation film 44 formed over the inner surface of an uppertransparent glass substrate 42. The lower orientation film 43 is formedover the SiN protective film 12 of the pixel formed on the lowertransparent glass substrate 41. A black mask 45 for shading, a colorfilter 46, an organic protective film 47, a transparent counter commonpixel electrode 48 and the upper orientation film 44 are superposed inthat order on the inner surface of the upper transparent glass substrate42. The transparent counter common pixel electrode 48 is opposite to thetransparent pixel electrode 8 of each pixel formed on the lowertransparent glass substrate 41. A common voltage Vcom is applied to thetransparent counter common pixel electrode 48. The color filter 46 isformed of an acrylic resin or the like and is colored with dyes indifferent colors and is formed in stripes across each pixel between thetwo adjacent image signal lines 102 (FIG. 11).

The organic protective film 47 prevents the dissolution of the dyes ofthe color filter 46 into the liquid crystal layer 40. The organicprotective film 47 is formed, for example, of a transparent resin, suchas an acrylic resin or an epoxy resin.

The lower transparent glass substrate 41 provided with the pixels, andthe upper transparent glass substrate 42 provided with the componentfilms, which are fabricated individually, are combined one over theother, and a liquid crystal is sealed in a space between the lowertransparent glass substrate 41 and the upper transparent glass substrate42 with a sealing member of an epoxy resin or the like attached to theperipheries of the glass substrates 41 and 42 to complete the liquidcrystal display. The transparent counter common pixel electrode 48 onthe upper transparent glass substrate 42 is connected to a lead, notshown, formed on the lower transparent glass substrate 41. The lead isformed by the same processes as those for forming the gate electrode 1,the source electrode 4 and the drain electrode 5. The orientation films43 and 44, the transparent pixel electrode 8 and the transparent countercommon pixel electrode 48 are formed in an area surrounded by thesealing member. Polarizing plates 49 and 50 are attached to therespective outer surfaces of the lower transparent glass substrate 41and the upper transparent glass substrate 42, respectively.

FIG. 10 is a fragmentary sectional view of a typical pixel of a liquidcrystal display in another preferred embodiment according to the presentinvention provided with a drain electrode 5, a source electrode 4 and animage signal line, not shown, formed by the taper etching techniques inaccordance with the present invention. After forming the firstconductive film A, i.e., the barrier layer, and the underlying layers bythe same processes as those explained in connection with the firstembodiment, an aluminum film of about 400 nm in thickness is formed bysputtering under conditions meeting the inequality: Tr>T>Tsd, thealuminum film is patterned by a known photolithographic process, andthen the workpiece is immersed in an etchant of a composition specifiedin the triangular diagram shown in FIG. 8 to form a source electrode 4,a drain electrode 5 and an image signal line of upward tapered crosssection having side surfaces inclined at an inclination of 45° or below.Then, a SiN protective film 12 is formed. The source electrode 4, thedrain electrode 5 and the image signal line of upward tapered crosssection improves the flatness of the SiN protective film 12 and hencethe flatness of the lower orientation film, so that the lowerorientation film can be uniformly rubbed and the liquid crystal isoriented uniformly to improve the display characteristics of the liquidcrystal display.

The gate electrode 1 may be formed of a material other than aluminum,prepared by adding one or more metals to aluminum as a matrix. Theadditive metals suppress the formation of aluminum hillocks andwhiskers. The material of the gate electrode 1, desirably, has a lowresistivity and must be capable of forming an insulating film in aself-aligned mode. Therefore, the additive metals must meet suchrequirements. The resistance of the gate electrode 1 can be reduced byproperly determining the additive metal contents.

Since the insulating film is formed by an anodic oxidation process, itis important to select metals which can be easily anodized. Metals ofthe groups IVa and Va are suitable metals meeting those requirements.Representative suitable metals are aluminum alloys containing at leastone of Ta and Ti. Such an aluminum alloy facilitates the control of thedeposition temperature and can be deposited in an excellent film ofcolumnar crystal grains.

Forming the gate electrode 1, the gate insulating film 2b, the scanningsignal line, the source electrode 4, the drain electrode 5 and the imagesignal line extending from the drain electrode 5 of the same materialenables etching films for forming those components with the sameetchant, which facilitates the control of the manufacturing processes.

FIG. 12 shows a liquid crystal display system incorporating the liquidcrystal display of the present invention. The liquid crystal displaysystem comprises a display unit 200, and an information processing unit220, such as a work station, a personal computer or a word processor.The display unit 200 comprises a liquid crystal display panel 202, alight source 201, a light regulating circuit 203, a control circuit 204comprising an image data generator 204A and a timing signal generator204B, a contrast regulating circuit 216, a storage capacitor drivingvoltage generator 205, and a common electrode driving voltage generator206. A liquid crystal module comprises a liquid crystal panel 217, asignal circuit 207 that generates a signal voltage, and a scanningcircuit 208 that generates a scanning voltage. The liquid crystal panel217 comprises TFTs 211 using amorphous Si films or polycrystalline Sifilms, storage capacitors 212, liquid crystal capacitors 213, and signallines 210 and scanning lines 209 for driving the TFTs 211. The voltageVstg generated by the storage capacitor driving voltage generator 205and the voltage Vcom generated by the common electrode driving voltagegenerator 206 are applied to a common storage capacitor driving line 215and a common electrode terminal 214, respectively. The voltages Vstg andVcom may be of the same level and of the same phase and are not subjectto any particular restrictions.

The liquid crystal display of the present invention can be applied tothe display units of desk-top computers and lap-top computers. Since thefeatures of the liquid crystal display of the invention are particularlyeffective when the liquid crystal display is applied to a computer whichneeds to have a lightweight construction, the liquid crystal display ofthe present invention is effective in constructing lightweight compactcomputers including notebook type computers. The liquid crystal displayof the present invention is applicable as a light shutter for aprojection display. The projection display comprises a liquid crystaldisplay and a projection unit including an optical system, convertsinput video signals into image signals of a signal system suitable forthe liquid crystal display, such as noninterlaced R-, G- and B- digitalsignals, by a video signal processing unit, and displays imagesrepresented by the digital signals on the liquid crystal display. Theimages displayed on the liquid crystal display are focused on a screenby the optical system. The light shutter is a principal factordominating the size of the optical system. The use of the liquid crystaldisplay having numerous pixels in a panel of a small area is effectivein miniaturizing the light shutter. The liquid crystal display of thepresent invention is applicable to miniature color monitors and largewall type TV sets.

According to the present invention, the gate electrodes, the insulatingfilms covering the gate electrodes and the scanning signal lines areformed in upward tapered sectional shapes having side surfaces inclinedat an inclination of 45° or below by etching metal films of a columnarcrystal structure, whereby the flatness of the protective film and thepicture quality can be improved. The present invention improves theyield of the drain electrodes, the source electrodes and wiring linesincluding the image signal lines. The upward tapered sectional shape ofthe gate electrodes suppresses electrostatic focusing when forming thegate insulating films, which make it possible to form stable insulatingfilms and improves the yield of the manufacturing process. Aluminum andaluminum alloys containing at least one of the transition elements ofthe groups IVa and Va are easily deposited in films of columnar crystalgrains facilitating the formation of lines having an upward taperedsectional shape. The inclination of the side surfaces of the lines canbe adjusted by properly determining the nitric acid concentration andthe acetic acid concentration of the etchant.

What is claimed is:
 1. A liquid crystal display comprising:a firstsubstrate provided with a plurality of scanning signal lines, aplurality of image signal lines arranged perpendicularly to the scanningsignal lines, a plurality of thin-film transistors each formed at arespective cross-over point of the scanning signal lines and the imagesignal lines, and a plurality of pixel electrodes connected respectivelyto the thin-film transistors; a second substrate disposed opposite tothe lower substrate and provided with a common electrode opposite to thepixel electrodes; and a liquid crystal layer sealed in a space formedbetween the first substrate and the second substrate; the scanningsignal lines being formed on the first substrate so as to form gateelectrodes of the corresponding thin-film transistors, respectively, thescanning signal lines being metal films of columnar crystal grains, thecolumnar crystal grains being crystal grains of metal in the form ofcolumns, the columns extending from the first substrate, andself-aligned oxide films being formed on the surfaces of the scanningsignal lines.
 2. A liquid crystal display according to claim 1, wherethe metal films have a tapered cross-section.
 3. A liquid crystaldisplay according to claim 2, wherein said metal films are films formedby deposition on the first substrate, the first substrate being at atemperature T during the deposition, and wherein Tr>T>Tsd, where Tr isthe recrystallization temperature of the metal and Tsd is the surfaceself-diffusion temperature of the metal.
 4. A liquid crystal displayaccording to claim 2, wherein an angle of the tapered cross-section withthe first substrate is at most 45°.
 5. A liquid crystal displayaccording to claim 4, wherein an angle of the tapered cross-section withthe first substrate is in a range of more than 20°, to 45°.
 6. A liquidcrystal display according to claim 2, wherein the metal films are madeof an aluminum alloy containing at least one of Ta and Ti.
 7. A liquidcrystal display according to claim 2, wherein each of the scanningsignal lines and the self-aligned oxide films has upward taperedsectional shapes having side surfaces inclined at an inclination of 45°or below, respectively.
 8. A liquid crystal display according to claim2, wherein the metal films for forming the scanning signal lines areformed of a material containing aluminum as a matrix, and at least oneof the transition elements of the element groups IVa and Va.
 9. A liquidcrystal display according to claim 2, wherein drain electrodes andsource electrodes of the thin-film transistors are formed of a metalfilm of columnar crystal grains in an upward tapered sectional shapehaving side surfaces inclined at an inclination of 45° or below.
 10. Aliquid crystal display according to claim 7 or 9, wherein theinclination of the side surfaces of the upward tapered sectional shapesis 20° or above.
 11. A liquid crystal display comprising:a firstsubstrate provided with a plurality of scanning signal lines, aplurality of image signal lines arranged perpendicularly to the scanningsignal lines, a plurality of thin-film transistors each formed at arespective cross-over point of the scanning signal lines and the imagesignal lines, and a plurality of pixel electrodes connected respectivelyto the thin-film transistors; a second substrate disposed opposite tothe lower substrate and provided with a common electrode opposite to thepixel electrodes; and a liquid crystal layer sealed in a space formedbetween the first substrate and the second substrate; the scanningsignal lines being formed on the first substrate so as to form gateelectrodes of the corresponding thin-film transistors, respectively, thescanning signal lines being formed from a metal film of columnar crystalgrains, wherein the metal film for forming the scanning signal lines isformed of a material containing aluminum as a matrix, and at least oneof the transition elements of the element groups IVa and Va, andself-aligned oxide films being formed on the surfaces of the scanningsignal lines.